[CentOS] Error trying to mount NFS volume from kickstart in CentOS 5

The post section of the kickstart configuration file I have been  
using in CentOS4 looks like this:

   %post
   mount centosmirror:/var/www/html/ks /mnt
   /mnt/ks/post/post.sh > /root/post_install.log 2>&1
   umount /mnt

I tried to do my first CentOS 5 kickstart installation today using an  
identical post section, but the post install script did not run.   
After instrumenting the kickstart a bit, I found that the above mount  
command failed with the following error message:

   mount.nfs: Input/output error

Not a very helpful error message -- any clues as to what is going  
on?  It's a little tricky and time consuming to debug kickstart  
problems like these, so I thought I'd ask the list and hopefully save  
some time.  If I execute that mount command after rebooting the  
system, it works just fine.

Thanks,
Alfred